1. Field of the Invention
The present invention relates to a multi-functional sensor and, more particularly, pertains to a high-sensitivity capacitive oil deterioration and level sensor adapted to generate signals indicative of oil deterioration and oil level.
2. Description of the Related Art
One of the most important factors that contributes to the efficiency and durability of internal combustion engines is lubrication. As engine oil "breaks down" it is less effective in protecting an engine from damage caused by friction between engine parts. More specifically, the normal life span of motor oils is limited by thermal breakdown, additive depletion and carbon particulates that result from the combustion process.
The deterioration of engine oil is marked by a decrease in the viscosity of the oil. The dielectric constant of engine oil provides an indication of the oil's deterioration or lack thereof. The dielectric constant of motor oil is typically between 1.6 and 3.2 depending upon its brand and age. For example, the dielectric constant of a particular brand of motor oil may increase from 2.19 to 3.2 after 400 hours of use in a particular internal combustion engine under certain operating conditions. Thus, it is desirable to have a means for indicating when engine oil has deteriorated to the point where it should be changed.
Premature lubrication failure can also result from the presence of contaminants in the engine oil such as coolant (glycol ethylene), fuel or water. The presence of these contaminants in motor oil is often indicative of a mechanical failure such as a damaged head gasket or a broken piston ring. Water and engine coolant have dielectric constants of approximately 87.5 and 37.0, respectively. The introduction of such contaminants into the engine oil significantly increases the dielectric constant of some of the fluid which circulates through the engine for the purpose of lubrication. Thus, it is also desirable to have a means for detecting the presence of these substances in engine oil.
Lubrication is also critical in applications which do not involve combustion but where oil is still monitored for degradation, e.g., compressors, pumps, and gearboxes. Thus, it is also desirable to have a means for detecting lubricant deterioration or the presence of foreign elements in lubricating oil.
U.S. Pat. No. 5,540,086 to Park et al. discloses an oil deterioration sensor adapted to monitor changes in the dielectric constant of oil. The disclosed oil deterioration sensor includes a capacitor geometry consisting of two flat, circular plates separated by a fixed gap. One of the plates is a ceramic disk upon which gold has been deposited to form an electrode. The other capacitor terminal, the ground electrode, consists of a portion of the sensor housing. This geometry works very well in most applications. However, this is not so in some cases, such as gearboxes, in which the change in dielectric constant is very small resulting in excessive gain requirements on the amplification circuit. Thus, it is also desirable to have a means for detecting small changes in the dielectric constant of oil.
Today's luxury automobiles rely heavily upon control systems which receive input signals from a wide variety of sensing devices. To ensure proper functioning of these control systems, a large number of highly dependable and accurate electronic transducers are needed to provide the aforementioned input signals. However, the implementation of too many transducers has proven to be uneconomical for automobile manufacturers. Thus, a single sensor including a plurality of transducers and their associated signal conditioning electronics is also desirable.
The dielectric constant of oil is also influenced by the temperature of the oil and by the specific formulation of a given brand of oil. Thus, it is also desirable to provide a multi-functional sensor with a means for compensating for changes in the dielectric constant of oil resulting from these factors.
Accordingly, an object of the present invention is to provide a high-sensitivity capacitive oil deterioration and level sensor which is adapted to detect small changes in the dielectric constant of oil.
Another object is to provide a high-sensitivity capacitive oil deterioration and level sensor which employs a cylindrical capacitor electrode geometry to generate signals indicative of a degree of oil deterioration and a level of the oil within the sensor.
Another object is to provide a high-sensitivity capacitive oil deterioration and level sensor with a capacitor geometry which eliminates the need for costly gold plating of electrodes.
Another object is to provide a high-sensitivity capacitive oil deterioration and level sensor including a reference capacitor which partially compensates for the difference in dielectric constant of different oil brands and for any geometric changes caused by thermal expansion of materials comprising the sensor, and to cancel stray capacitances arising in the wiring of the electrodes to circuitry within the sensor.